Legal requirements are, for instance, further reducing the allowable boundary values for nitrogen oxides in the exhaust gas of internal combustion engines. To meet these requirements, adding ammonia (NH3) to the exhaust gas in order to convert the nitrogen oxides (NO and NO2) contained in it into the environmentally safe compounds nitrogen (N2) and water (H2O) is known.
Essentially, ammonia or ammonia precursors such as urea or ammonium carbamate can be made available for treatment of motor vehicle exhaust gas by means of a special tank. To make available an amount of ammonia sufficient for the service life of the vehicle, this tank must be very large. But generally there is not enough room for this purpose. When using smaller tanks it is necessary to refill them in the interim. It should be noted when filling tanks that this activity must be performed by specialized automotive service centers in order to preclude poisoning or odor nuisance.
Moreover, ammonia can, however, also be produced directly on board the motor vehicle. Thus, documents DE 103 37 901 A1 and DE 199 03 533 A1 disclose methods for synthesis of ammonia by means of a plasma-catalytic reactor located in the exhaust gas line of the internal combustion engine. These synthesis methods are however comparatively complex, fault-susceptible and moreover expensive.
Finally, document DE 196 54 977 C2 discloses a method for reducing nitrogen oxides and black smoke. For this reason, in a diesel engine operated lean, water is injected into the combustion chamber. Vaporizing the injected water reduces the combustion temperatures so that fewer nitrogen oxides are formed. The water to be injected is obtained there from the exhaust gas of the diesel engine.
Furthermore, document DE 196 48 219 C1 discloses a device for separating water from combustion exhaust gases. The water obtained in this manner can be used in the most varied ways, as a result of which carrying a separate water tank in the vehicle can be eliminated.
Against this background, the object of this invention is to make available an improved method for producing ammonia for treating exhaust gas in internal combustion engines in a motor vehicle. Moreover, the object of the invention is also to devise a device for implementing the method.